The present invention relates to optical storage media, and, in particular, to improving storage densities in such media by means of lasers that operate at short wavelengths.
There is a pressing need to improve information storage densities in optical media. Currently, optical storage systems use lasers with a wavelength of .about.780 nm. A blue laser with a wavelength of 400-500 nm could improve storage density four fold because its focused spot size would be substantially smaller than the 780 nm laser's.
When the first optical memory systems were developed, it was understood that a compact laser source in the blue region of the spectrum could increase storage density. However, compact blue laser sources do not yet exist. Existing blue laser sources are approximately the size of a desktop computer (See Verdeyen, Laser Electronics, 2nd Ed., (Englewood Cliffs, N.J., 1989) at 334). To use an existing blue laser source in an optical storage system doubles the size of the computer, making the storage system too bulky for desktop applications. Also, existing blue lasers require substantial air cooling, thereby further restricting their use in optical storage systems.
Thus there exists a need for a compact blue laser source. With the large market for ultra-high-density memory storage in a compact system, the development of compact blue lasers is of high priority.
There are three methods for creating a compact source for blue laser light. The first is to fabricate a semiconductor chip that provides the required feedback to an amplifier section whose signal gain is in the blue region of the spectrum. Laser devices of this type that emit blue light have too short a lifetime at room temperature for use in an optical memory system ("The End of the Beginning", Compound Semiconductor (July/August 1995) at 35).
The second method requires a high-power semiconductor laser and a doubling crystal. The emission from the high-power diode has a wavelength from 800-1000 nm. This light interacts with the crystal to produce a photon with a frequency twice the frequency of the pump photons. Laser emission results when the doubling crystal is placed in an optical cavity that supplies feedback in the blue region of the spectrum (Goldberg, et al., "Blue light generation in bulk periodically field poled LiNbO.sub.3 ", 31 Electronics Letters 1576-1577 (1995)).
This method is impractical for three reasons. First, since the crystal length is typically less than two centimeters, the number of blue photons created per pass through the crystal is small. Hence this method is too inefficient for practical application. Second, the optical system required to generate blue light comprises mirrors. For optimized operation, the mirrors must be within 0.001 mm of the correct position; making the system environmentally unstable. Third, the size of the high-powered diode laser, the electronics to drive it, and the large heat sink required to maintain it in stable operation make this system too large for a desktop optical storage system.
A third method for generating blue laser light uses as the gain material an optical fiber doped with a rare earth ion. A fiber made from heavy metal fluorides is the only fiber host that has shown any significant emission in the blue region of the spectrum. This fiber, called ZBLAN in its preform stage, is comprised of fluorides of zirconium, barium, lanthanum, aluminum, and sodium. To generate blue laser light, this fiber can be doped with either trivalent praseodymium (Pr.sup.3+) or trivalent thulium (Tm.sup.3+).
Praseodymium has exhibited lasing at 491 nm when pumped by infrared laser diodes (Baney, et al., "Blue Pr.sup.3+ -doped ZBLAN fiber upconversion laser", 21 Optics Letters 1372-1374 (1996)). However, Pr.sup.3+ dopant yields strong emissions at other visible wavelengths (red, orange, and green), all of which share the same upper laser level. Thus doping with Pr.sup.3+ does not produce blue light as efficiently and with as much power as does doping with Tm.sup.3+.
Blue lasers of Tm:ZBLAN fiber have been created using a single-laser diode pump at a wavelength between 1120 nm and 1160 nm (Sanders, et al., "Laser diode pumped 106 mW blue upconversion fiber laser", 67 Applied Physics Letters 1815-1817 (1995)). This single-wavelength pumping scheme suffers from two significant drawbacks. First, creating laser diodes that operate in the 1120 to 1160 nm region is very difficult. Second, the absorption of pump photons cannot be optimized at a single pump wavelength. Upconversion to the energy level of a blue light laser requires three different transitions, and none of them resonates at the single wavelength. Thus the threshold for lasing rises, demanding higher output from the pump laser. Also, a small deviation in the output wavelength of the pump laser reduces substantially the performance of the blue laser.
Therefore the need for a compact blue laser system cannot be satisfied by any apparatus or method currently available.